Porous polyethylene hollow fiber membranes are conventionally prepared by: melt-spinning polyethylene through a spinneret equipped with a double nozzle; cooling the spun hollow fiber; annealing, cold-stretching and hot-stretching the resulting hollow fiber; and heat-setting the stretched hollow fiber.
During the cooling step, the melt-spun polyethylene hollow fiber is crystallized, and when the fiber is subjected to stress along the direction of the drawing, micro-fibrils are formed lengthwise on the fiber, to form lamella stacks. Amorphous regions are present between lamella stacks, and the stretching step opens up the amorphous region to render the hollow fiber porous. When the cooling rate is rapid, the size of final pores becomes small, and when slow, comparatively large.
Thus, the cooling step in the manufacture of a hollow fiber membrane is critical in determining the pore size and form, and the characteristics of the porosity of a final hollow fiber membrane.
Hitherto, the cooling has been generally performed by introducing a cooling air stream maintained at room temperature to the outer wall surface thereof. Such a cooling procedure produces a hollow fiber membrane having a uniform pore-size distribution throughout the thickness of the membrane (uniform-porosity hollow fiber membrane). However, such a uniform pore-sized hollow fiber membrane is not satisfactory in terms of water permeability and particle cut-off property.
Meanwhile, U.S. Pat. No. 5,294,338 (Kamo et al.) discloses a porous polyethylene hollow fiber membrane having rectangular pores, and a hydrophilized porous hollow fiber membrane obtained by coating a hydrophilic polymer on the outer surface of said hollow fiber membrane. The hydrophilic coating layer of the hydrophilized hollow fiber membrane, however, undesirably gets abraded with several uses.